The present invention relates to scanning thermal microscopy; and more particularly, to a resistance-based probe which is used for mapping spatial variation of the thermal properties of a surface, such as temperature, thermal conductivity, and thermal diffusivity as well as being used for detecting various chemical reactions and phase transformations taking place within the studied sample.
More particularly, the present invention relates to a high resolution scanning thermal probe which includes a nanometer sized filament structure formed at the end of an AFM-type cantilever where the force is detected by either optically measuring the deflection or by means of an integrated piezoresistive element.
Further, the present invention relates to a free-standing nanometer sized probe for thermal measurements having decreased thermal conductivity and thermal capacitance thus insuring faster response time, higher frequencies in the active measurement mode (when the probe is heated), and improved spatial
Still further the present invention relates to a four legged thermal probe and a method for producing the four legged thermal probe as well as to a four points resistance measurement technique which results in the elimination of contact potential and contact resistivity thus increasing both the temperature sensitivity and the signal-to-noise ratio of the thermal measurements.
Scanning thermal microscopy is a near field technique which permits mapping of spatial variations of thermal properties of a sample, such as temperature, thermal conductivity and diffusivity with sub-micrometer resolution. This type of microscopy has been applied to the study of thermal properties of polymers and pharmaceuticals, locally induced phase transformations, and spatially resolved photothermal spectroscopy as well as other scientific areas. With the continued reduction of the size of integrated circuits, temperature mapping of electronic and optoelectronic devices has become increasingly more important to optimize heat dissipation in the circuits and to identify phase modes caused by local xe2x80x9chot spotsxe2x80x9d.
Various types of probes with different heat sensitive elements, including thermocouples, contact potentials, Joule expansion elements, Schottky diodes, and resistance based transducers have been developed over the last few years. Prior art systems include thermal probes which may be a resistive probe consisting of a wire making point contact with a sample for scanning the sample surface. In a passive mode of the measurement when no heat is applied to the probe, the temperature of the sample is measured by monitoring the change in the resistivity of the wire. While in the active mode, the sample is locally heated by applying alternating electric current to the wire, thus allowing measurement of thermal conductivity and thermal diffusivity of the sample. Additionally induced local changes such as phase transformations or chemical reactions in the sample may be measured. Due to the fact that the measurement involves heat flow from the sample to the probe, a large thermal resistance of the probe is required in order to improve the accuracy of the thermal measurement. Disadvantageously, prior art thermal probes such as, for example, TM Microscopes Cantilever conventionally used in resistance based transducers, use a sensitive element consisting of a five micron diameter Platinum Rhodium wire which, due to its large dimensions, is unable to provide a high spatial resolution measurement. Additionally, such a wire has undesirably low thermal resistance and high thermal capacitance which decreases the accuracy of the measurements and deteriorates the sensitivity as well as the response time of the temperature measurement.
Building or fabricating a freestanding nanometer sized probe would advantageously decrease both the thermal conductance and the capacitance, thus insuring faster response time and higher frequencies in the active mode of measurements, as well as providing an improvement in the spatial resolution of the probe.
As described in U.S. Pat. No. 5,171,992, nanometer scale probes for magnetic measurement are produced by an electron beam chemical vapor deposition (CVD) process in which a substrate is placed in an evacuated chamber within an electron beam unit, and a volatile organometallic compound gas stream is introduced into the sub-chamber at the same time an electron beam is initiated.
The electron beam impinges upon an upper surface of the substrate and causes decomposition and preferential deposition of the decomposed product of the organometallic gas onto the surface of the substrate. Such deposition occurs within the region irradiated by electron beam. Some deposition also occurs outside the region irradiated by the electron beam due to electron scattering from the surface of the substrate. As the process continues, additional layers of the deposited decomposed components of the organometallic gas continue to build up thereby creating a needle like structure. A conical tip shape for the created needle and its shank diameter are achieved by control of the primary beam voltage and the beam""s Gaussian profile. The fabricated needle is covered by a magnetic metal layer to allow the intended magnetic measurements. By manipulating the electron beam, two and three dimensional needle tip structures may be fabricated.
Although the technique described in U.S. Pat. 5,171,992 permits production of nanometer scale probes, the resulting probes are not suited for thermal measurements and are not applicable to four point thermal measurement techniques.
It is therefore an object of the present invention to provide a mechanically stable nanometer scale thermal probe adapted for thermal measurements, having a high spatial resolution, fast response time, high thermal resistance and high signal-to-noise ratio.
It is another object of the present invention to provide a technique for producing a multi-leg nanometer scale thermal probe enabling highly accurate measurements of temperature, thermal conductivity and thermal diffusivity of a sample, as well as inducing local changes such as phase transformations and/or chemical reactions.
It is still a further object of the present invention to provide a four point thermal measurement technique employing a four leg nanometer scale probe through which a current (AC or DC) is applied to two legs of the probe and the voltage drop indicative of a temperature value is measured by contacting the opposite two legs, thus eliminating contact resistance, improving temperature dependence of the resistance and eliminating the error in the temperature readings introduced by temperature gradients along the filament wire.
According to the teachings of the present invention, a thermal nanometer scale probe for thermal measurements of a sample includes an AFM-type cantilever where the force is detected by either optically measuring the deflection or by means of an integrated piezoresistive element. The AFM-type cantilever includes a conductive patterned layer (preferably Au) formed on the surface, and a multi-leg nanometer scale filament structure deposited on the electrically isolated segments of the conductive patterned layer of the AFM-type cantilever. The multi-leg nanometer scale filament structure includes a plurality of legs, a bridge portion, and a contact tip positioned in the center of the bridge portion. Each leg of the multi-leg filament structure has a bottom end contiguously engaging a respective one of the plurality of electrically isolated segments of the conductive patterned layer on the AFM-type cantilever. A top end of each leg of the multi-leg filament is joined with the top ends of other legs of the filament structure by the bridge portion from which the contact tip extends into point contact with the measured sample.
Although the multi-leg nanometer scale filament structure may include two and three legs, a three-dimensional four legged structure is preferred due to its mechanical rigidity, its ability to use a four point measurement technique, low thermal conductivity and fast time response as well as improved signal-to-noise ratio and high spatial resolution of the measurements.
The electrically isolated segments of the conductive (preferably gold) patterned layer on the AFM-type cantilever are electrically separated by respective gaps cut, etched or otherwise formed in the surface of the conductive patterned layer at predetermined locations in order that each gap includes an undercut portion having sidewalls which are inaccessible to a conductive material deposited onto the surface of the conductive patterned layer thus eliminating a danger of creating a xe2x80x9cshortxe2x80x9d between the lower ends of the filament legs.
The conductive patterned layer is formed in close proximity to a front end of the cantilever, preferably within 10 micron distance from a front edge. The front edge of the cantilever is trimmed to form an angled front edge which aids in the deposition process when the cantilever structure is to be angled or inclined with respect to the axis of the electron beam.
As another aspect of the present invention, such provides a method for producing a thermal nanometer scale probe including the steps of:
microfabricating a cantilever,
forming a patterned conductive layer in proximity to the front edge of the cantilever,
cutting or otherwise forming a plurality of gaps in the patterned conductive layer to develop electrically isolated segments of the patterned conductive layer, and
growing a multi-leg nanometer scale filament structure having a plurality of legs, a bridge portion, and a contact tip positioned substantially at the center of the bridge portion.
The growth of the nanometer scale filament structure is conducted by a focused electron beam chemical vapor deposition technique including the following operational steps:
(a) exposing the cantilever to a precursor gas,
(b) directing an electron beam to a predetermined location on each of the electrically isolated segments of the pattern conductive layer in a predetermined sequence for a predetermined time duration to permit the formation of a conductive deposit at the predetermined locations on each of the electrical isolated segments thus forming lower ends of the legs of the nanometer scale filament structure,
(c) sequentially changing a relative disposition between the electron beam and the cantilever thus growing the legs of the filament structure by depositing the conductive deposits starting from the lower ends of the legs and continuing through a plurality of successive points until the upper ends of the legs are joined to each other by the bridge portion extending over the gaps which are formed in the patterned conductive layer, and
(d) directing the electron beam substantially to the center of the bridge portion to grow the contact tip of the filament structure.
Where the filament structure has four legs, two crossing gaps are cut into the patterned conductive layer on the surface of the cantilever thus defining four electrically isolated segments. The filament structure may be grown in a number or variety of ways. In one fabrication technique the electron beam may be directed substantially perpendicular to the cantilever and scanned in predetermined successive steps of predefined time duration (starting with the predetermined locations on each of the four electrically isolated segments) either in a clockwise or counter-clockwise fashion in order to grow the four legs in a quasi-simultaneous manner.
Alternatively, only two legs at a time of the four leg structure may be grown in the quasi-simultaneous manner, changing precursor gases before depositing the other two legs. In this manner, a structure with pairs of legs formed of different materials may be formed, thus producing a thermocouple structure.
In still another technique, the filament structure may be grown by maintaining the electron beam substantially stationary and tilting the cantilever a predetermined angle relative to the electron beam. When one leg of the filament structure is grown, the cantilever is turned in a manner whereby the cantilever structure is angled with respect to the electron beam and the second leg is grown until the upper ends of these two legs are joined together. Since the two legs of the filament structure are grown independently each from the other, the legs may be grown from two different materials thus forming a structure similar to a thermocouple.
Usually the total deposition time for growing the filament structure is in the range of 2-6 minutes, thus allowing filament growth having leg lengths in the range of 2-5 microns, with a diameter of each leg in the range of 30-100 nm. The end radius of a contact tip grown in the center of the bridge portion of the filament structure is smaller than 20 nm providing high resolution of the temperature measurements by the thermoprobe of the present invention.
It has been found that the growth rate for the deposit decreases during electron beam deposition (e.g., the amount of material added for a given electron beam exposure) decreases with the distance from the base point at the substrate, causing a xe2x80x9cbendingxe2x80x9d of the deposited structure. This effect can be corrected by increasing the exposure time with each successive step, in order to account for the decreased growth rate. In this manner substantially straight legs may be grown, if desired.
Still further, the present invention provides a method for 2-point and 4-point thermal measurement of a sample by which a nanometer scale thermoprobe is brought into contact with a sample to be measured. The nanometer scale thermoprobe includes an AFM-type cantilever integrated with a filament structure having four legs, a bridge portion joining the upper ends of the legs, and a contact tip extending from the center of the bridge portion for contiguous point contact with a sample to be measured. In the 2-point measurement, an electric current is applied to two legs, and a voltage drop is measured at the legs. In the 4-point thermal measurement technique, an electric current is applied to first and second legs of the filament structure and a voltage drop indicative of a sample temperature is measured at a third and fourth leg of the filament structure.
The measurement can be conducted in two modes, namely: passive mode when the temperature measurements are conducted by monitoring the thermoprobe temperature scanning the thermoprobe over the surface of the sample; and in an active mode by heating the thermoprobe, monitoring the heat flow between the thermoprobe and the sample to be measured and deriving the thermo conductivity and thermodiffusivity of the sample from the obtained data.
These and other features and advantages of the subject invention will be more fully understood from the following detailed description of the accompanying Drawings.